please let me know when you done the extrapolation and how you did it i have just finished the movement again. iam now doing it the way described, when there i currently no new position incoming, the player just keeps moving into the last known direction a little bit slower (not noticable). this works great, only thing what can happen is that sometimes you make a little extra curve but that doesnt really influence anything.

i always move the client player a little bit slower than the real player to make shure he doesnt always reach the position close.
this way the clients are 1/2 second delayed in the internet i would say. a test yesterday evening showed that with this method no lags occur at all in the movement.
they also showed when 2 players are constantly moving with this system. (the server uses send_skill(my._receive_x, SEND_ALL | SEND_VEC | SEND_RATE | SEND_UNRELIABLE); to send the data with dplay_entrate = 4;
i have almost constantly 0.5kb/s upload at my network card. when only one player is moving theres 0.2-0.3 kb/s and if no on moves its around 0.1-0.3 kb/s wich i guess are the keepalive ping messages.

i made a little video to show the movement. this video is taken at the client its very slow due to the recording software so dont worry about that. just notice how the player moves at the client. this was taken in lan.

http://www.ackbytes.de/upload/movement.wmv

(850kb size of movie)

use videolan to watch it, have fun also notice the panel i made can be dragged around and it can be pinned so you dont accidently move it around. almost all hud elements in the game will be moveable this way.